The present description relates to an automatic transmission for automotive vehicles, particularly relates to an arrangement of planetary gear sets and frictional elements of an automatic transmission.
There is required a shifting gear mechanism having a plurality of planetary gear sets to achieve six or more forward speed ratios. In addition to the planetary gear sets, there are required at least five frictional elements, such as brakes and clutches, to achieve six forward and one reverse speed ratios. U.S. Pat. No. 6,884,197 presents one example of arrangements of the planetary gear sets and the frictional elements.
In the '197 patent, as shown particularly in its FIG. 12, one of three planetary gear sets has a sun gear permanently fixed to a transmission case. In other words, the sun gear does not rotate at all. Its ring gear is permanently connected to an input shaft, and rotates together with this input shaft. So, this planetary gear set permanently reduces speed of the input shaft, and it will be called a permanent reduction planetary gear set. A clutch is arranged overlapped with the permanent reduction planetary gear set in the axial direction. Another clutch is arranged overlapped with another planetary gear set next to the permanent reduction planetary gear set. Then, additional clutch and brakes are arranged offset from the three planetary gear sets in the axial direction. Therefore, the arrangement of the '197 patent may cause the input shaft longer, its support structure such as a transmission casing larger, and eventually the overall size of the automatic transmission greater. This larger size may become a constraint in mounting the automatic transmission on an automotive vehicle. Especially when the automatic transmission is laterally mounted on the vehicle, the shaft length may be needed to be shorter.
Therefore, there is a need for an improved arrangement of planetary gear sets and frictional elements of an automatic transmission of the type presented in the prior art.